The International pane in System Preferences (left) lets you choose the preferred languages for applications to use. You can manually remove language files from applications via the Languages section of the Get Info window (right) for each application.
Perhaps even more importantly, all applications read available fonts at launch. This means that reducing the number of fonts can have a two-fold benefit, though it is probably most dramatically seen on older and slower Macs and may not be overtly noticeable for more recent models (unless you have an inordinate amount of fonts installed).
You can remove or disable fonts using the Font Book application included with Mac OS X (which can also be used to preview any installed fonts). Disabling a font will increase performance (as the font is essentially ignored by the computer) without removing it completely. Removing a font completely will free up space as well as improve performance.
To disable a font, select it in the font list and click the button below the list that looks like a checkbox. To re-enable the font, select it and click the same button. To delete a font, select and press the delete key on the keyboard.
You'll notice that fonts are organized by collection and family. A font family contains one or more variations (such as regular, bold, light and italic) of a single font. Removing a single member of the family removes only that variation, not all versions of the font. Font collections are organized groups of fonts that you can use to quickly locate specific fonts. The Collection column also includes an All Fonts collection, system-generated collections for specific languages and collections labeled Computer and User.
The Computer collection lists fonts installed for use by all users of the computer (those located in the /Library/Fonts folder at the root level of the hard drive). The User collection lists fonts installed for use only by the currently logged in user (which are located in the /Library/Fonts folder in each user's home folder). There may be overlap between these, depending on whether specific fonts were installed for a single user or for all users. If there is overlap, you can remove fonts from the User collection.
9. Find and remove large files and folders
This may seem like an obvious solution to slimming down the contents of your hard drive. In fact, finding and removing large files wherever possible is the best and most obvious way to recover space. The trick here is finding those large files. Files created by applications in obscure locations on your hard drive, documents that have gone from one Mac to the next over the course of several years, or folders that you just assume don't contain as much as they do are all reasons to take a good look at which files are taking up large amounts of hard drive space.